Which do you prefer and why? I go back and forth and obviously couldn’t use both! [name_f]Mary[/name_f] [name_f]Rose[/name_f] would be a double first name or a first/middle combo, not sure yet.
[name_f]Rosemary[/name_f] is still uncommon, but it is more popular then [name_f]Mary[/name_f] [name_f]Rose[/name_f]. I think that [name_f]Mary[/name_f] [name_f]Rose[/name_f] would be a good first and middle name combo. In my opinion, [name_f]Rosemary[/name_f] has a better flow for a first name.
I like both options, but I have a lot of love for [name_f]Rosemary[/name_f]. We actually have it on our list as a middle name for our favorite girl name, [name_f]Ada[/name_f] 
To me, [name_f]Rosemary[/name_f] has a classic, vintage charm to it. It isn’t terribly common anymore.
[name_f]Mary[/name_f] [name_f]Rose[/name_f] is incredibly sweet and a solid choice. My worry with this one would be that, should you chose to make both names the first name, [name_f]Rose[/name_f] would get left off or shifted to the middle spot. I also think it is difficult to find a middle name that sounds good with a double-barrel first name…it sort of begins to sound like laundry-listing after a while.
namergirl3, [name_f]Rose[/name_f] would technically be the middle name but she would be called [name_f]Mary[/name_f] [name_f]Rose[/name_f]. I like [name_f]Mary[/name_f] and [name_f]Rose[/name_f] alone too, so really nothing bad could happen with shortening.
I love [name_f]Rosemary[/name_f]!